Hi,
The bandwidth of OFDM is ideally infinite.
The channel bandwidth and FFT size alone don’t provide enough information to describe the entire structure of the OFDM signal. Recall the following relationship:
Δf=fs/N
Δf=fs/N
where Δf is the subcarrier spacing, fs is the sample rate used at the modulator input, and N is the FFT size.
For 802.11a, however, the FFT size is typically 64; with a sample rate of 20 MHz, this yields a subcarrier spacing of 312.5 kHz. Of these 64 potential subcarriers, 12 of them are unused (sometimes referred to as “virtual carriers”). Of the remaining 52 subcarriers, 4 contain pilot tones, so only 48 subcarriers carry information at any given time.
Thanks